Note, this is not a dupe of https://issues.jenkins-ci.org/browse/JENKINS-16998 which affects downstreamParameterized.

I believe this is actually just a mistake in the documentation (wiki).

The docs mention downstream() accepting 'ALWAYS' as a valid value for the second argument, but this is not accepted by the job-dsl implementation and is not an option provided in the jenkins UI (radio button texts are: "Trigger only if build succeeds", "Trigger even if the build is unstable", "Trigger even if the build fails").

Docs:
https://github.com/jenkinsci/job-dsl-plugin/wiki/Job-reference#downstream

DSL Code:
https://github.com/jenkinsci/job-dsl-plugin/blob/master/job-dsl-core/src/main/groovy/javaposse/jobdsl/dsl/helpers/publisher/PublisherContextHelper.groovy#L476

UI screenshot is attached.